The invention relates in general to semiconductors and in particular to rapid thermal processing of semiconductor devices.
During fabrication of a semiconductor device such as a transistor, N-type and P-type regions are formed in a wafer made of semiconductor material. High velocity streams of gases containing dopant atoms are directed onto the wafer, causing dopant atoms to be implanted below the wafer's surface. Donor-type dopant atoms such as phosphorus are used to form the N-type regions, and acceptor-type dopant atoms such as boron are used to form the P-type regions. The wafer is heated to temperatures approaching 1000.degree. C., allowing the dopant atoms to diffuse into the crystalline matrix of the semiconductor material and occupy free lattice spaces. The dopant atoms that occupy the free lattice spaces become electrically active.
The electrical characteristics of the semiconductor device are dependant upon how precisely the N-type and P-type regions are formed. Precise depth, length and width of the regions allow the device to operate according to specification. When the dimensions of a region change, the electrical characteristics of the device deviate from the specification. The step of heating the wafer is critical to precise formation of the N-type and P-type regions. For example, if too high a temperature is applied to the wafer, or if the wafer is heated for too long a period of time, the dopants will diffuse more than they should. As a result, the dimensions of the regions will change and the electrical characteristics of the semiconductor device will deviate.
The wafer can be heated in an annealing furnace. Activation temperatures ranging between 800.degree. C. and 1000.degree. C. can be achieved within 30 minutes. The problem with furnace annealing is that the long heating period allows the donor-type and acceptor-type dopant atoms to diffuse and the dimensions of the N-type regions and P-type regions to change.
In the alternative, the wafer can be heated by rapid thermal processing (RTP). RTP is performed with a Halogen lamp and a parabolic reflector. The Halogen lamp generates broadband radiation, and the parabolic reflector focuses the broadband radiation onto the wafer. Activation temperatures can be reached in less than one minute. Thus, RTP can reach the activation temperatures far more quickly than furnace annealing.
One difficulty encountered with Halogen lamps is achieving and maintaining temperature uniformity and controllability across the wafer. Only certain wavelengths in the IR spectrum can be used to heat the wafer. Yet the Halogen lamp generates many different wavelengths at random, and intensities of the useful wavelengths fluctuate. The wavelengths range from ultraviolet to visible to infrared. The wavelength distribution is not uniform and cannot be controlled. Consequently, the rise and fall times of the activation temperature cannot be controlled precisely.
Mere usage of the lamp also contributes to variations in the wavelength distribution. Thus, constant usage of the Halogen lamp diminishes the ability to control the activation temperature. Additionally, the parabolic reflector itself contributes to the lack of controllability. Design of the parabolic reflector is not precise; it is determined empirically.
Achieving and maintaining temperature uniformity and controllability is further complicated by the different types of materials (e.g. photoresist, dopants) on the surface of the wafer. Heating of the different materials is very sensitive to the optical reflectivity and thermal conductivity of the semiconductor material. As a result, the same RTP apparatus could heat different wafers to different temperatures.
This lack of controllability and uniformity of temperatures during RTP affects the ability to form precise regions in the wafer.